This most often is related to a sync error with your Microsoft Account.
First try a Soft Reset. Hold Volume Down + Power buttons until the phone vibrates and reboots.
If you still can't download make sure your account can sync: Settings > email + accounts > hold down n the account name > tap "sync" from the pop up menu.
If sync still fails:
- make sure date and time are correct for where you are
- un-select any restrictions in Data Sense
- try visiting your Microsoft Account on a PC browser and setting a new password - then back on the phone go to Settings > email + accounts > hold down n the account name > tap "sync" from the pop up menu > enter new password at prompt
